Heterogeneous catalysts are used in a vast number of chemical and petrochemical processes. In many cases, the viability of the process depends on the successful combination of the activity of the catalyst and its selectivity and stability. A catalyst that has a high activity but exhibits poor selectivity to the desired products might not be useful to implement a chemical reaction in a commercial scale. Furthermore, a catalyst having a good activity and a good selectivity to the desired product, but showing a poor stability may not be suitable for industrial application. An optimum balance between activity, selectivity and stability must be achieved in order to consider the practical application of a catalyst.
Practical application of catalysts is also limited by economy and scalability of their preparation methods. In the scientific literature are described a number of catalysts showing acceptable performance in terms of activity, selectivity and stability, but their preparation methods are often impracticable outside a chemical laboratory or simply not economically viable in an industrial application.
Small metal or metal oxide particles having diameters in the nanoscale range are often referred to as clusters. There is an important motivation to research the catalytic properties of metal or metal oxide clusters because they differ to a great extent from the properties exhibited by bulkier particles. It is often the case that unexpected catalytic effects can be attributed to the action of clusters.
There is an advantage in supporting catalytically active metal-containing clusters on zeolitic materials. Zeolitic materials are unique supports for metal clusters because the steric restrictions imparted by their cages and pores limit the size of the clusters that can form in them. The restrictions imparted by the apertures (often termed “windows”) between cages and pores limit the size of what can enter and leave the pores and cages. Thus clusters can be formed from small precursors (e.g. metal salts) in the cages and be trapped there.
The cages of zeolitic materials are small enough to exert solvent-like effects on clusters formed within them and thus the cages may induce different catalytic properties to the clusters they contain. Confinement of clusters in zeolitic material cages hinders cluster interactions and aggregation and thereby increase cluster stability.
Supported metal and metal oxide cluster catalysts can be prepared in a number of different ways. U.S. Pat. No. 4,552,855 describes a preparation method which is stated to produce zero-valent metallic clusters supported on zeolites. The deposition of the metal takes place by vaporisation of the metal at a high vacuum.
Alternative methods of producing supported metal cluster catalysts involve the impregnation of the support with metal-carbonyl complex precursors. An example of such a preparation method is described in U.S. Pat. No. 4,192,777.
U.S. Pat. No. 5,194,244 describes compositions comprising a zeolite and an alkali-metal compound wherein the sum of the amount of the alkali-metal in the compound plus any metal cation exchanged into the zeolite is in excess of that required to provide a fully metal cation-exchanged zeolite. Once the compounds are loaded into the zeolite, they are calcined to produce at elevated temperature to form a basic material that can be used as a basic catalyst, or as an adsorbent. Haber et al, in Pure and Applied Chemistry, vol 67, Nos 8/9, pp 1257-1306, discuss deposition-precipitation as a method of forming supported catalysts (section 2.1.2.2), in which an active metal is deposited onto a carrier in a precipitating solution, by slow addition or in situ formation of a precipitating agent. It is noted that for a porous support, deposition takes place preferentially in the external parts.
U.S. Pat. No. 4,113,658 describes a deposition-precipitation process for preparing materials comprising finely divided particles of metallic materials substantially homogeneously deposited on a nucleating surface such as silica. This is achieved by preparing a suspension of the nucleating surface, and crystallising metal compound onto the surface at nucleation sites from a solution comprising the metal compound.
EP 2 314 557 describes a catalyst for production of lower olefins from synthesis gas, using a catalyst in which iron has been deposited on a support that is chemically inert towards iron, such as alumina.
Promoters are chemical species added to solid catalysts or to processes involving catalysts in order to improve their performance in a chemical reaction. By itself, a promoter has little or no catalytic effect. Some promoters interact with active components of catalysts and thereby alter their chemical effect on the catalysed substance. The interaction may cause changes in the electronic or crystal structures of the active solid component. Commonly used promoters are metallic ions incorporated into metals and metallic oxide catalysts, reducing and oxidizing gases or liquids, and acids and bases added during the reaction or to the catalysts before being used.
Potassium is a well-known promoter of Group VIII-metal catalysts, commonly used in iron based High Temperature Fischer-Tropsch (HTFT) catalysts. Potassium, however, facilitates the sintering of the Group VIII metals and metal oxides. For example, U.S. Pat. No. 6,653,357 describes the effect of potassium migration in the Fischer-Tropsch process. The deactivation due to promoter migration is of special relevance if the promoter is a poison for a secondary catalytic function in bi-functional catalysts, for example in a hydrocarbon synthesis process using a hydrocarbon synthesis catalyst and an acidic catalyst, as described for example in U.S. Pat. No. 7,459,485. High loading of potassium may also lead to activity losses due to blocking of the pores of the support, and in some applications it has been shown that promotional effects deteriorate at potassium loadings exceeding 2% in weight.
Another problem associated with the preparation of supported metal catalysts is the tendency of the metals to aggregate or sinter during use, or during the any high temperature pre-treatment that may be required for activation. Such aggregation or sintering reduces the effective surface area of catalyst available for the catalysed reaction, which reduces catalyst activity
It is desirable to provide a metal or metal oxide catalyst that has long term stability, and to provide a method for making such a catalyst, that avoids problems such as sintering, and also migration of active catalyst components during synthesis or use that can lead to catalyst deactivation.